This invention relates to humidifying systems for fuel cells, and more particularly to a humidifying system that utilizes a hollow fiber membrane to humidify fuel gases and oxidant gases each supplied to the fuel cell.
Polymer electrolyte fuel cells have been attracting widespread attention in recent years as being a power source for electric vehicles, and the like. The polymer electrolyte fuel cell (PEFC) can generate power at ordinary temperatures, and is thus finding various practical applications.
In general, a fuel cell system includes a fuel cell divided by a solid polymer electrolyte membrane into a cathode and an anode in such a manner that the solid polymer electrolyte membrane is sandwiched between the cathode and anode provided at each side of the membrane. The cathode is supplied with air containing oxygen and the anode is supplied with fuel gases containing hydrogen. A chemical reaction then takes place between oxygen in the supplied air and hydrogen in the supplied fuel gases, thereby generating electric power, which is used to drive an external load.
Efficiency in power generation of the fuel cell system depends upon several important parameters, which include ionic conductivity of hydrogen ions migrating across the solid polymer electrolyte membrane. The higher ionic conductivity, the more hydrogen ions may migrate across the solid polymer electrolyte membrane for each unit hour, thus increasing electric power generated through the electrochemical reaction.
Keeping the ionic conductivity high however requires the solid polymer electrolyte membrane to remain humidified so as not to dry up; therefore, every fuel cell system includes a humidifier without exception.
Among usable humidifiers are an ultrasonic humidifier, a nozzle-jetting humidifier, a steam humidifier, and the like, but most commonly used is a humidifier employing a hollow fiber membrane because of its lower power consumption and compact body that serves to save space for installation.
An exemplified humidifying system for a fuel cell including such a humidifier as conventionally designed is illustrated in FIG. 11.
Referring to FIG. 11, a fuel-cell humidifying system 100 principally includes a fuel cell 101, two humidifiers 102, 103, an ejector 104, and a supercharger (S/C) 105. The fuel cell 101 includes an anode 101a and an cathode 101c, and allows hydrogen in fuel gases supplied to the anode 101a and oxygen or oxidant gases in the air supplied to the cathode 101c to react with each other, thereby generating electric power. The humidifiers 102, 103 humidify gases supplied to the anode 101a and the cathode 101c in the fuel cell 101 by transferring moisture containing in cathode exhaust gases discharged from the cathode 101c of the fuel cell 101. The ejector 104 serves to circulate and supply fuel gases to the anode 110a. The S/C (supercharger) 105 supplies air as oxidant gases to the cathode 101c. Hereupon, it is to be understood that the fuel cell 101 is included as a component in the humidifying system.
A description will be given of operation of the fuel-cell humidifying system 100 as described above.
In operation, fuel gases having no or low moisture, which have been regulated at a specific pressure in a regulator 106 and supplied to the ejector 104, are supplied to the humidifier 102 after passing through the ejector 104. The fuel gases (low-moisture gases) supplied to the humidifier 102 are humidified, while passing through a humidification module in the humidifier 102, by the cathode exhaust gases (moisture-rich gases) discharged from the cathode 101c of the fuel cell 101. Thereafter, the humidified fuel gases are supplied to the anode 101a. Hydrogen in the fuel gases fed to the anode 101a in the fuel cell 101 reacts with oxygen in the air supplied from the S/C (supercharger) 105 to the fuel cell 101, and generates electric power. Fuel gases unused in the reaction in the fuel cell 101 are supplied as anode exhaust gases to a subsequent process (e.g., for use in a catalytic combustor). Part of the anode exhaust gases is sucked by the ejector 104, and is recirculated as fuel gases.
On the other hand, air (as low-moisture gas) in the atmosphere is sucked and pressurized in the S/C (supercharger) 105, and supplied to the humidifier 103.
The air (low-moisture gas) supplied to the humidifier 103 is humidified, while passing through the humidification module, by the cathode exhaust gases (moisture-rich gases) discharged from the humidifier 102, and is then supplied to the cathode 101c. Air unused for reaction with hydrogen in the fuel gases in the fuel cell 101 is first supplied to the humidifier 102 as cathode exhaust gases (moisture-rich gases). The cathode exhaust gases supplied to the humidifier 102, while passing through the humidification module in the humidifier 102, provide moisture to humidify the fuel gases supplied from the ejector 104 to the humidifier 102, and are discharged from the humidifier 102. The cathode exhaust gases discharged from the humidifier 102 are then supplied to the humidifier 103, and provide moisture to humidify air supplied from the S/C (supercharger) 105 to the humidifier 103, while passing through the humidification module in the humidifier 103. The cathode exhaust gases discharged from the humidifier 103 are supplied to a subsequent process (e.g., for use in catalytic combustor).
However, the conventional fuel-cell humidifying system 100 as described above has disadvantages as follows:
(1) Fuel gases to be supplied to the fuel cell 101 need keeping relative humidity thereof constant. However, as chemical reaction in the fuel cell intrinsically generates heat, increase in output of the fuel cell 101 raises heat of reaction in proportion to the output, which inevitably results in increase in temperature of exhaust gases discharged from the fuel cell 101. Therefore, in the humidifier 102 that humidifies fuel gases to be supplied to the fuel cell 101, high-temperature cathode exhaust gases containing excessive moisture, i.e., supersaturated (having high vapor pressure) would be provided to humidify the fuel gases, and would thus make the fuel gases excessively humidified (raising a dew point thereof) beyond a target dew-point range required in the fuel cell 101, as shown in FIG. 12. Consequently, the continuous operation of humidifying the fuel gases with the cathode exhaust gases could disadvantageously cause a flooding phenomenon in which a passage of gases is clogged with water collected in a gap formed along the solid polymer electrolyte membrane.
(2) On the other hand, in the humidifier 103 that humidifies air to be supplied to the fuel cell 101, humidifying operation in the humidifier 102 previously performed by supplying the cathode exhaust gases, which would have excessively humidified the fuel gases, would cause a shortage of moisture, and would thus lower a dew point of the air below the target dew-point range for the fuel cell 101, as shown in FIG. 12. Consequently, the solid polymer electrolyte membrane of the fuel cell 101 would disadvantageously dry up, and would thus hinder stable generation of electric power.
The inventors made an attempt to avoid excessive humidification of the fuel gases by varying the length and number of the hollow fiber membranes provided in the humidifier 102; however, such operation could only make a straight line indicating the relationship between dew points and outputs of the fuel cell 101 translate vertically, but disadvantageously could not lay the straight line down to a horizontal position. To make matters worse, when the output of the fuel cell 101 gets higher, another disadvantageous situation would develop contrariwise in which sufficient humidification could not be achieved.
The present invention has been created to eliminate the above-discussed disadvantages.